The invention relates to an arrangement for transferring information between the secondary and the primary circuit of a transformer. The arrangement finds particular utility in transferring control information needed in voltage stabilization across a transformer used in a switched-mode supply.
Switched-mode circuit solutions for providing supply voltage to electronic circuits are very popular because of their relatively good efficiency. A switched-mode circuit always needs an inductive component, usually a transformer, to store energy in a magnetic field and to transfer it further to the load. The primary winding of the transformer is connected to the feeding source of energy and the secondary winding to the load. The energy supplied to the transformer by the primary winding must be controlled according to the load. This requires that the voltage of the load be monitored and a signal dependent of said voltage be transferred to the primary side of the transformer to control the current in the primary winding. In order to minimize the occurrence of malfunctions and to improve electrical safety, galvanic isolation is provided between the secondary and the primary circuits of the feedback.
Arrangements are known from the prior art that include a separate component or unit for the galvanic isolation mentioned above. One such known structure is depicted in FIG. 1. It comprises a power transformer 10, switch unit 101, secondary circuit 102, control unit 103 and an isolating unit 104. The transformer 10 comprises a primary winding w1, secondary winding w2 and a ferromagnetic core depicted in FIG. 1 by vertical lines drawn between the windings. The primary winding w1 belongs to a circuit that further includes said switch k and a source of energy which has a certain source voltage Vi. The switch k is used to xe2x80x9cchopxe2x80x9d the current il in the primary winding. When the switch is closed, energy is stored in the magnetic field of the transformer. When the switch is open, energy is discharged from the magnetic field of the transformer to the secondary circuit 102. In the simplified structure of FIG. 1 the secondary circuit comprises a rectifier diode D followed by a filter capacitor C and a load connected to the terminals of said filter capacitor. Load voltage Vo is sensed by the control unit 103 the output of which is connected to the input of the isolating unit 104. The output signal vc of the isolating unit is directed to the switch unit 101 controlled by it. The feedback is arranged such that the load voltage Vo follows relatively faithfully a reference voltage generated at the control unit.
The input and output sides of the isolating unit 104 are galvanically isolated from each other. The method of isolation is not specified in FIG. 1. The isolation may be realized optically, for example, in which case the isolating unit comprises light emitting and receiving components, among other things. The drawback of this solution is that the feedback is relatively slow, which may result in stability problems in voltage regulation. Inductive isolation is also known, in which case the isolating unit comprises a transformer for that purpose. The drawback of this solution is that a separate isolating transformer results in a considerable increase in production costs.
An object of the invention is to reduce said disadvantages associated with the prior art. A structure according to the invention is characterized by what is specified in the independent claim 1. Some advantageous embodiments of the invention are specified in the other claims.
The basic idea of the invention is as follows: Control information is transferred to the primary circuit of a power transformer by means of auxiliary windings in the same transformer. The auxiliary windings are arranged in pairs in the transformer core branches in such a manner that the magnetic fluxes corresponding to the control signal will not induce a voltage in the energy-transferring windings, and the voltages induced by the magnetic flux corresponding to the energy transfer are opposite both in the primary and secondary-side winding pairs. Thus the transfer of control signal and the transfer of energy will not disturb each other. The transformer may be advantageously realized as a planar structure on a printed circuit board.
An advantage of the invention is that it facilitates an arrangement of the transfer of control information to the primary circuit of a power transformer at relatively low production costs. This is made possible by the fact that the necessary auxiliary windings have a small number of turns and can be arranged without an additional stage in the production process. Another advantage of the invention is that it facilitates fast feedback for a switched-mode structure, for example. A further advantage of the invention is that the arrangement according to it is functionally reliable.